Vibratory classifying screens are old and well known. Typical classifying screens are shown in Voith U.S. Pat. No. 288,187, granted Nov. 27, 1883; Howard U.S. Pat. No. 857,942, granted June 25, 1907; Rice U.S. Pat. No. 1,034,302, granted July 30, 1912; Wildhaber U.S. Pat. No. 1,331,303, granted Feb. 17, 1920; Seaman U.S. Pat. No. 1,141,727, granted June 1, 1915; Bell U.S. Pat. No. 1,174,657, granted Mar. 7, 1916; Bell U.S. Pat. No. 1,291,371, granted Jan. 14, 1919; Cobb U. S. Pat No. 1,491,583, granted Apr. 22, 1924; Reynolds et al U.S. Pat. No. 1,991,001, granted Feb. 12, 1935; Wettlaufer U.S. Pat. No. 2,029,848, granted Feb. 4, 1936; Hinklf U.S. Pat. No. 2,106,742, granted Feb. 1, 1938; Gilson U.S. Pat. No. 2,358,453, granted Sept. 19, 1944; Ward U.S. Pat. No. 2,384,715, granted Sept. 11, 1945; McDonell U.S. Pat. No. 2,399,280, granted Apr. 30, 1946; Missien U.S. Pat. No. 2,730,236, granted Jan. 10, 1956; Aker U.S. Pat. No. 2,748,942, granted June 5, 1956; Van Zelst U.S. Pat. No. 2,862,620, granted Dec. 2, 1958; Parks U.S. Pat. No. 3,044,624, granted July 17, 1962, U.S. Pat. No. 3,045,824, granted July 24, 1962, and U.S. Pat. No. 3,081,875, granted Mar. 19, 1963; Tonjes et al U.S. Pat. No. 3,098,037, granted July 16, 1963; Greenwell U.S. Pat. No. 3,109,808, granted Nov. 5, 1963; Loock et al U.S. Pat. No. 3,367,497, granted Feb. 6, 1968; Tonjes et al U.S. Pat. No. 3,367,498, granted Feb. 6, 1968; Tonjes U.S. Pat No. 3,481,468, granted Dec. 2, 1969; Frangos U.S. Pat. No. 3,640,386; Ljunggvist U.S. Pat. No. 3,804,246, granted Apr. 16, 1974; and Gundlach U.S. Pat. No. 4,233,151, granted Nov. 11, 1980. In these devices the trays are in a fixed position in relation to each other.